Clarify principle that features of external resources are not authoritative

[azaroth42]

Add a sub-section in section 2 that we provide information about resources not under our control as hints to the client. Clients should always trust the canonical source of any data rather than a third party description. This includes especially created, creator, modified, rights, format, language, textDirection, processingLanguage of resources, and all of the properties of Agents.

Discussion: http://www.w3.org/2016/08/05-annotation-irc#T15-30-52 (above)